1. Technical Field
The present invention relates to a piston ring for an internal combustion engine or for a compressor, in particular a piston ring having a groove that is arranged on the running face and has a periodically varying width and a varying, variable depth in the circumferential direction.
2. Related Art
Modern, large-volume engines for ships are still two-stroke diesel engines, since this type of engine can be designed in such a manner that the speed thereof is typically in a range from approximately 50 rpm to 250 rpm (typically less than 100 rpm) and the power thereof can reach up to approximately 100 MW, depending on the number of cylinders. Such large-volume, slow-running two-stroke ship engines preferably act directly on the drive shaft(s) of the propeller(s), since a reduction gear to reduce the rotation speed can be omitted owing to the speed of said engines.
Such large-volume two-stroke engines typically have two separate oil circuits, one for engine lubrication and one for cylinder lubrication. Cylinder lubrication ensures that enough lubricant is provided at a suitable point in time to guarantee sufficient lubrication of the cylinder surfaces and piston rings.
The cylinder lubricant is injected through the liner into the piston chamber, depending on the load of the machine. The piston rings run on this lubricating film, the supporting surface. Here it is a matter inter alia of injecting as little lubricant as possible in order to save costs and prevent over-lubrication. Cylinder lubrication takes place for example in the upper third of the stroke, by supplying lubricant by means of a lubricant pump through lubricant inlets, which are for example provided in a plane in the cylinder wall, into the cylinder so that the lubrication of the piston and of the piston ring is ensured in as optimal a manner as possible. The oil supply into the cylinders usually takes place using the gas counter pressure method.
For example, a lubricant injection system can be used that injects lubricant into the cylinders via nozzles in a precisely metered manner. A computer-controlled system registers the position in which a piston is located and then supplies lubricant in a targeted manner. This takes place at high pressure, so that the lubricant is sprayed very finely in order to obtain the most uniform possible wetting of the cylinder liner, but targeted to where the piston rings are and where the friction actually takes place.
If one considers that modern, large-volume two-stroke ship engines are operated at a speed of approximately 50 rpm to 250 rpm with a stroke of up to 2500 mm, the time span available for the supply of the lubricant and the distribution of the supplied lubricant is short and presents great challenges in ensuring the quality of the lubrication. If one assumes for example that a cylinder has an (inner) diameter of 900 mm and 8 inlets for the oil supply are provided distributed uniformly around the circumference of the cylinder wall, the supplied lubricant must be distributed in the circumferential direction over a length of approx. 350 mm starting from the respective inlets in the time span available.